I pay for the pro flickr account, so I basically use Flickr as my backup, as I upload full scale images. My wife is a semi-professional photographer, so we still backup all her RAW images on CDs (maybe DVDs someday).
A while back we had a discussion about putting your photos online and backing them up, etc. I found this program tonight, it allows you to backup the images you have uploaded to Flickr.
The only downside to this is that when I put pics onto Flickr, I downgrade them so they don't take so long to load. But, I also backup my images. So, I suppose if I lost the images this would come in handy.